I've also had the same experience with a taxi driver - though not in London. I was going with my mum in a taxi to the eye clinic where she lives (in Suffolk). Somehow the conversation got round to the fact that the driver had moved from London several years before. He then started to slag London off - how it had been taken over by immigrants, what a dump it was, etc., etc. I informed him that my neighbours were from all different parts of the world and we all got along pretty well.
Like Mishap I do think that people who suddenly "flip" in this way have got mental health problems.
I think racism is different from prejudice. A postman said, in the comments following the article, that he had been called "Pinky" and "Snowflake" by his black colleagues. This could be construed as prejudice but I don't think it can be described as racism because I believe racism is prejudice + power. If people are prejudiced against, that is unpleasant, but if that prejudice is then translated into discriminatory practices, for instance in the judicial system, the employment market, the education system, etc., etc., that is racism.